Oil and gas wells are expensive to construct, and it is advantageous to operate these wells as efficiently as possible. One way of providing for an increased efficiency in the operation of wells is to place equipment downhole in the well bore under the control of other equipment located on the surface. The equipment can be measurement sensors which supply useful information for the subsequent working of the well, for example data regarding pressure, the nature of the solids and fluids encountered, the temperature, etc. The equipment can be other controllable or monitoring equipments which supply important orders from the surface to control various parameters of the well or the reservoir with equipment and device such as valves, protective covers, etc. It is therefore important to be able to transmit information from the surface to various downhole equipments. Several prior art methods have attempted to provide electrically or electromagnetically communications between the surface equipment and the downhole equipment.
Traditionally, some prior art systems have placed cables in the well bore to provide communications and also power to the downhole equipment. Safely and accurately placing the cables within the well bore along side of the piping structure or string is difficult and time consuming to achieve. In addition, this requires additional equipment to be used increasing the costs associated with the well. Well bores are a harsh environment, and numerous failure mechanisms exist that cause the reliability of such systems to be unacceptably low. Furthermore, a cable incorporating such sensors, or being connected to control devices, located at a substantial depth cannot be lowered in every situation. The installation of such cable is possible when completing the well, but become practically impossible when the well is completed. In particular, the cable may not be able to be lowered when valves or separation devices cannot be crossed by a cable, whether or not the cable is fitted with sensors.
Some other prior art systems have attempted to use wireless communication system, relying upon the inherent coaxial nature of the well bore and the piping structure or tubing string disposed within the bore. These prior art systems however, typically provide a higher frequency data signal. These systems typically use toroidal coils or ferromagnetic choke assemblies placed on the piping structure or strings to provide a sufficiently large series impedance to the data signals to electrify a predefined portion of the piping structure or string. U.S. Pat. No. 4,839,644 describes such a method and system for wireless communications in a cased borehole having a tubing string.
Other prior art systems are based on transmission of electromagnetic waves guided by metal tubing; this transmission system is more particularly described in U.S. Pat. No. 5,394,141 (FIG. 1). A transmitter 3 is located downhole in the well and applies electric signals between two points 1 and 2 on a metal tubing 4. The electric signal can flow trough the metal tubing 4, the casing 5 or even the conductive fluid 6 filling the well; but due to sufficient impedance of the metal tubing the electric signal is transmitted to a surface transceiver 8. However, the required sufficient impedance largely depends on the geometric characteristics of the well and on the impedance of the surrounding environment: filling fluid, metal tubing, casing, formation, etc. It is better to limit or control the dependency of those parameters. For example, if the resistivity of certain layers is inadequate, as is the case with certain sedimentary, tertiary, pericontinental rocks like those of the North Sea or the Gulf of Mexico, the attenuation can become excessive along the well, which makes it impossible to use such a device in most offshore wells unless it is possible to accept a drastic reduction in the transmitted information flow.
Therefore, it would be advantageous to provide an improved system for wireless communication in a well bore not depending of all these parameters.